Tom's Yard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tom's Yard clear sky chart from 2005-11-06 to 2024-04-03.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	16% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	78857
	Night	16% 9% 4 7% 6% 3 4 5% 5% 7% 35%	81269
Transparency	Day	49% 7% 12% 19% 11% 2	78567
	Night	47% 7% 12% 21% 12% 2	81078
Seeing	Day	25% 16% 25% 19% 11% 4	13938
	Night	22% 13% 25% 28% 12% 1	23580

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 16% 6% 10% 6% 3 4 3 3 5% 28%	2553
	11th to 20th	Day	12% 18% 8% 13% 8% 3 4 4 4 4 21%	2840
	21st to end	Day	19% 18% 6% 9% 7% 4 5% 3 4 4 19%	3124
February	1st to 10th	Day	16% 15% 5% 8% 8% 3 5% 4 5% 5% 26%	1710
	11th to 20th	Day	15% 10% 5% 10% 7% 4 4 3 6% 5% 29%	1663
	21st to end	Day	19% 9% 4 11% 5% 3 5% 4 5% 4 30%	1550
March	1st to 10th	Day	15% 11% 4 6% 6% 4 4 5% 4 6% 36%	2085
	11th to 20th	Day	18% 11% 4 7% 6% 3 4 4 5% 6% 32%	2090
	21st to end	Day	19% 9% 4 6% 6% 3 4 4 5% 6% 34%	2304
April	1st to 10th	Day	17% 10% 4 6% 7% 3 5% 4 4 6% 34%	2184
	11th to 20th	Day	16% 10% 3 5% 6% 3 4 5% 5% 6% 37%	2134
	21st to end	Day	22% 8% 3 6% 5% 3 5% 4 5% 5% 35%	2227
May	1st to 10th	Day	19% 7% 3 7% 6% 3 6% 5% 5% 8% 31%	2340
	11th to 20th	Day	22% 6% 3 5% 5% 3 5% 4 5% 7% 33%	2418
	21st to end	Day	15% 7% 3 5% 5% 3 5% 5% 5% 9% 39%	2725
June	1st to 10th	Day	14% 6% 3 5% 6% 4 6% 6% 6% 8% 36%	2520
	11th to 20th	Day	14% 6% 2 6% 6% 4 6% 6% 7% 9% 35%	2520
	21st to end	Day	12% 4 2 5% 6% 4 6% 6% 7% 10% 39%	2520
July	1st to 10th	Day	10% 5% 2 6% 6% 4 6% 6% 7% 8% 38%	2520
	11th to 20th	Day	7% 5% 3 6% 6% 5% 6% 6% 7% 9% 40%	2467
	21st to end	Day	10% 6% 3 7% 7% 4 7% 6% 7% 9% 35%	2547
August	1st to 10th	Day	10% 5% 2 6% 7% 5% 7% 7% 8% 10% 33%	2327
	11th to 20th	Day	12% 6% 2 6% 6% 4 6% 6% 6% 9% 36%	2282
	21st to end	Day	13% 5% 2 6% 6% 3 5% 6% 6% 10% 38%	2376
September	1st to 10th	Day	18% 5% 2 5% 5% 3 4 4 5% 6% 43%	2160
	11th to 20th	Day	14% 5% 2 4 4 2 4 4 5% 7% 47%	2138
	21st to end	Day	17% 7% 2 5% 5% 3 5% 5% 6% 6% 38%	1890
October	1st to 10th	Day	18% 6% 2 6% 6% 3 4 4 4 6% 40%	1800
	11th to 20th	Day	18% 6% 3 6% 6% 3 5% 5% 5% 7% 37%	1800
	21st to end	Day	19% 8% 3 6% 5% 3 4 4 5% 6% 37%	1980
November	1st to 10th	Day	12% 7% 3 5% 4 4 4 4 6% 7% 44%	1764
	11th to 20th	Day	17% 9% 4 6% 6% 3 5% 6% 5% 7% 34%	1577
	21st to end	Day	18% 10% 4 7% 6% 3 4 4 6% 6% 32%	1520
December	1st to 10th	Day	19% 12% 5% 9% 6% 4 5% 5% 5% 6% 25%	1520
	11th to 20th	Day	18% 14% 5% 6% 5% 3 4 4 5% 6% 29%	1910
	21st to end	Day	21% 16% 5% 9% 9% 4 4 5% 5% 4 18%	2772
January	1st to 10th	Night	14% 16% 6% 9% 7% 4 4 3 5% 6% 26%	4388
	11th to 20th	Night	16% 15% 7% 10% 7% 3 4 4 4 5% 24%	4120
	21st to end	Night	23% 14% 5% 10% 6% 3 4 4 4 5% 22%	4532
February	1st to 10th	Night	16% 10% 5% 7% 7% 3 5% 5% 5% 6% 30%	2470
	11th to 20th	Night	16% 9% 4 8% 7% 4 5% 5% 5% 7% 30%	2403
	21st to end	Night	16% 10% 4 8% 6% 4 4 4 5% 5% 32%	1903
March	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 6% 6% 40%	2090
	11th to 20th	Night	17% 10% 3 6% 5% 3 4 4 5% 5% 38%	2090
	21st to end	Night	17% 7% 3 7% 5% 4 5% 3 4 5% 38%	2293
April	1st to 10th	Night	19% 10% 4 7% 6% 3 4 5% 5% 6% 32%	1821
	11th to 20th	Night	15% 7% 2 6% 6% 3 3 5% 5% 8% 39%	1775
	21st to end	Night	18% 6% 2 6% 5% 3 5% 5% 5% 7% 39%	1746
May	1st to 10th	Night	18% 9% 3 5% 5% 3 5% 6% 5% 7% 34%	1620
	11th to 20th	Night	18% 7% 3 5% 6% 2 5% 5% 5% 8% 37%	1474
	21st to end	Night	11% 7% 2 5% 4 4 4 5% 6% 10% 41%	1550
June	1st to 10th	Night	12% 6% 3 5% 5% 3 4 5% 6% 7% 46%	1440
	11th to 20th	Night	12% 5% 2 7% 5% 3 6% 5% 7% 9% 38%	1440
	21st to end	Night	10% 4 2 4 5% 4 5% 5% 5% 12% 44%	1440
July	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 6% 9% 44%	1440
	11th to 20th	Night	6% 4 3 4 5% 3 5% 5% 6% 10% 48%	1488
	21st to end	Night	10% 4 3 5% 5% 4 5% 5% 7% 10% 42%	1771
August	1st to 10th	Night	7% 5% 2 5% 7% 3 5% 4 7% 11% 43%	1611
	11th to 20th	Night	11% 7% 3 6% 5% 3 4 5% 6% 7% 45%	1599
	21st to end	Night	12% 6% 2 5% 4 4 4 5% 6% 9% 44%	1980
September	1st to 10th	Night	15% 5% 2 6% 5% 3 4 3 5% 6% 45%	1800
	11th to 20th	Night	12% 5% 2 4 4 3 4 5% 4 7% 51%	1818
	21st to end	Night	15% 6% 3 6% 5% 3 3 5% 5% 7% 42%	2070
October	1st to 10th	Night	17% 6% 2 4 4 3 3 5% 5% 6% 44%	2160
	11th to 20th	Night	18% 5% 3 4 4 3 4 4 5% 7% 42%	2160
	21st to end	Night	21% 8% 3 5% 4 2 4 5% 4 6% 38%	2376
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 4 5% 6% 50%	2311
	11th to 20th	Night	17% 9% 3 6% 5% 3 5% 5% 5% 6% 36%	2601
	21st to end	Night	16% 9% 3 6% 6% 3 5% 4 6% 6% 35%	2660
December	1st to 10th	Night	19% 11% 5% 7% 5% 4 5% 6% 5% 6% 27%	2660
	11th to 20th	Night	18% 11% 4 7% 6% 4 5% 5% 5% 6% 30%	3285
	21st to end	Night	19% 13% 5% 9% 7% 4 5% 5% 5% 6% 22%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	58% 6% 12% 18% 4 2	2554
	11th to 20th	Day	66% 4 10% 11% 8% 1	2840
	21st to end	Day	68% 5% 9% 13% 4 1	3124
February	1st to 10th	Day	59% 5% 13% 16% 7% 1	1710
	11th to 20th	Day	55% 6% 10% 21% 7% 1	1672
	21st to end	Day	56% 5% 11% 17% 8% 3	1550
March	1st to 10th	Day	49% 6% 13% 22% 10% 1	2085
	11th to 20th	Day	53% 5% 11% 18% 9% 3	2090
	21st to end	Day	51% 5% 10% 19% 12% 2	2304
April	1st to 10th	Day	51% 5% 11% 18% 13% 2	2184
	11th to 20th	Day	47% 7% 9% 18% 17% 2	2134
	21st to end	Day	50% 6% 8% 20% 13% 3	2227
May	1st to 10th	Day	51% 7% 10% 18% 13% 2	2340
	11th to 20th	Day	50% 7% 11% 16% 13% 3	2422
	21st to end	Day	43% 8% 16% 21% 11% 2	2725
June	1st to 10th	Day	44% 8% 12% 21% 14% 1	2520
	11th to 20th	Day	42% 11% 13% 19% 13% 2	2520
	21st to end	Day	39% 8% 14% 26% 13% 1	2520
July	1st to 10th	Day	41% 10% 12% 20% 14% 3	2450
	11th to 20th	Day	36% 9% 14% 23% 17% 1	2392
	21st to end	Day	42% 9% 12% 20% 16% 2	2405
August	1st to 10th	Day	41% 11% 15% 21% 11%	2314
	11th to 20th	Day	42% 10% 13% 22% 12% 1	2282
	21st to end	Day	40% 8% 13% 22% 15% 3	2376
September	1st to 10th	Day	41% 7% 10% 21% 16% 3	2160
	11th to 20th	Day	35% 6% 11% 27% 18% 3	2138
	21st to end	Day	44% 7% 11% 18% 18% 2	1890
October	1st to 10th	Day	45% 6% 12% 20% 15% 2	1800
	11th to 20th	Day	46% 7% 13% 23% 11%	1800
	21st to end	Day	48% 6% 12% 21% 13% 1	1980
November	1st to 10th	Day	38% 6% 15% 21% 16% 4	1764
	11th to 20th	Day	49% 6% 11% 20% 11% 2	1577
	21st to end	Day	52% 6% 11% 23% 8%	1520
December	1st to 10th	Day	59% 8% 13% 14% 6%	1520
	11th to 20th	Day	56% 5% 12% 21% 5%	1906
	21st to end	Day	67% 6% 12% 11% 4	2772
January	1st to 10th	Night	58% 6% 11% 18% 7% 1	4388
	11th to 20th	Night	61% 7% 12% 14% 6% 1	4120
	21st to end	Night	65% 5% 9% 16% 5% 1	4532
February	1st to 10th	Night	52% 7% 11% 21% 8%	2470
	11th to 20th	Night	53% 6% 11% 18% 12%	2405
	21st to end	Night	53% 6% 10% 18% 11% 2	1903
March	1st to 10th	Night	43% 6% 13% 24% 13% 1	2090
	11th to 20th	Night	47% 7% 11% 20% 13% 2	2090
	21st to end	Night	48% 5% 10% 20% 15% 3	2293
April	1st to 10th	Night	52% 6% 10% 17% 13% 2	1821
	11th to 20th	Night	42% 7% 11% 21% 16% 3	1775
	21st to end	Night	43% 8% 10% 21% 14% 3	1746
May	1st to 10th	Night	48% 8% 9% 19% 12% 4	1620
	11th to 20th	Night	45% 7% 12% 18% 15% 2	1475
	21st to end	Night	37% 9% 15% 24% 12% 3	1551
June	1st to 10th	Night	38% 7% 12% 26% 15% 2	1440
	11th to 20th	Night	39% 11% 13% 22% 14% 2	1440
	21st to end	Night	33% 9% 15% 27% 16% 1	1440
July	1st to 10th	Night	36% 10% 12% 23% 14% 4	1400
	11th to 20th	Night	30% 9% 15% 25% 19% 2	1440
	21st to end	Night	33% 11% 17% 21% 16% 3	1672
August	1st to 10th	Night	33% 10% 16% 25% 15% 1	1600
	11th to 20th	Night	37% 8% 14% 23% 16% 1	1599
	21st to end	Night	36% 8% 13% 24% 15% 3	1980
September	1st to 10th	Night	40% 6% 11% 22% 17% 4	1800
	11th to 20th	Night	32% 6% 10% 29% 21% 2	1818
	21st to end	Night	41% 7% 11% 22% 18% 2	2070
October	1st to 10th	Night	39% 8% 13% 24% 14% 1	2160
	11th to 20th	Night	40% 6% 12% 26% 14% 1	2160
	21st to end	Night	47% 5% 9% 24% 14% 1	2376
November	1st to 10th	Night	35% 5% 14% 25% 17% 3	2311
	11th to 20th	Night	48% 5% 10% 21% 14% 3	2601
	21st to end	Night	48% 6% 13% 21% 10% 1	2660
December	1st to 10th	Night	55% 8% 14% 17% 7%	2660
	11th to 20th	Night	55% 6% 12% 20% 8%	3288
	21st to end	Night	61% 7% 13% 15% 3	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 24% 26% 10% 3 1	191
	11th to 20th	Day	40% 22% 29% 7% 1	270
	21st to end	Day	31% 30% 29% 7% 3 1	297
February	1st to 10th	Day	34% 29% 23% 12% 3	270
	11th to 20th	Day	41% 28% 26% 5%	270
	21st to end	Day	38% 21% 25% 14% 1	362
March	1st to 10th	Day	35% 18% 24% 17% 5%	460
	11th to 20th	Day	35% 21% 25% 13% 4 2	460
	21st to end	Day	31% 23% 26% 13% 5% 2	506
April	1st to 10th	Day	34% 21% 23% 16% 4 2	428
	11th to 20th	Day	26% 19% 24% 18% 9% 5%	412
	21st to end	Day	32% 20% 26% 14% 6% 2	420
May	1st to 10th	Day	30% 19% 25% 14% 7% 4	423
	11th to 20th	Day	26% 19% 25% 18% 10% 2	447
	21st to end	Day	19% 15% 25% 19% 15% 7%	485
June	1st to 10th	Day	18% 16% 27% 22% 13% 3	450
	11th to 20th	Day	21% 14% 24% 25% 14% 3	450
	21st to end	Day	13% 13% 33% 26% 12% 3	450
July	1st to 10th	Day	15% 8% 25% 25% 19% 9%	445
	11th to 20th	Day	9% 6% 29% 29% 20% 7%	444
	21st to end	Day	14% 8% 29% 27% 18% 5%	488
August	1st to 10th	Day	13% 6% 26% 27% 20% 9%	446
	11th to 20th	Day	14% 7% 25% 26% 18% 10%	439
	21st to end	Day	15% 8% 26% 24% 17% 11%	495
September	1st to 10th	Day	21% 8% 19% 24% 20% 8%	450
	11th to 20th	Day	15% 7% 20% 28% 20% 9%	450
	21st to end	Day	25% 14% 24% 18% 14% 5%	450
October	1st to 10th	Day	24% 9% 20% 26% 17% 4	450
	11th to 20th	Day	24% 17% 25% 24% 10% 2	450
	21st to end	Day	26% 18% 28% 20% 8% 1	495
November	1st to 10th	Day	22% 15% 25% 22% 14% 1	410
	11th to 20th	Day	31% 19% 24% 17% 7% 1	237
	21st to end	Day	29% 24% 29% 14% 3 1	180
December	1st to 10th	Day	34% 25% 26% 7% 6% 2	180
	11th to 20th	Day	35% 19% 28% 14% 3	180
	21st to end	Day	32% 23% 24% 16% 5%	198
January	1st to 10th	Night	26% 22% 31% 18% 3	760
	11th to 20th	Night	29% 19% 28% 21% 3	760
	21st to end	Night	25% 21% 29% 22% 2	836
February	1st to 10th	Night	28% 16% 28% 25% 3	760
	11th to 20th	Night	30% 23% 29% 17% 1	743
	21st to end	Night	30% 18% 29% 21% 2	628
March	1st to 10th	Night	26% 18% 30% 22% 4	760
	11th to 20th	Night	29% 19% 26% 20% 6%	760
	21st to end	Night	24% 18% 32% 20% 6% 1	836
April	1st to 10th	Night	28% 18% 29% 20% 5%	729
	11th to 20th	Night	21% 15% 28% 27% 8% 1	711
	21st to end	Night	23% 13% 25% 31% 7%	666
May	1st to 10th	Night	22% 10% 22% 31% 13% 1	540
	11th to 20th	Night	20% 8% 29% 35% 8%	413
	21st to end	Night	13% 8% 21% 36% 19% 2	389
June	1st to 10th	Night	14% 6% 24% 38% 18%	360
	11th to 20th	Night	14% 6% 24% 37% 18% 1	360
	21st to end	Night	10% 3 28% 44% 14%	360
July	1st to 10th	Night	11% 4 17% 43% 23% 3	350
	11th to 20th	Night	7% 4 17% 41% 29% 2	396
	21st to end	Night	10% 3 19% 43% 24% 2	591
August	1st to 10th	Night	9% 2 14% 41% 32% 2	537
	11th to 20th	Night	13% 4 16% 35% 28% 4	535
	21st to end	Night	15% 3 18% 33% 28% 3	792
September	1st to 10th	Night	17% 4 17% 33% 26% 3	720
	11th to 20th	Night	14% 6% 19% 27% 30% 4	720
	21st to end	Night	18% 9% 22% 27% 21% 3	720
October	1st to 10th	Night	19% 7% 23% 27% 21% 3	720
	11th to 20th	Night	21% 13% 26% 28% 11% 1	720
	21st to end	Night	26% 12% 26% 28% 7% 1	792
November	1st to 10th	Night	16% 11% 28% 34% 11% 1	740
	11th to 20th	Night	26% 18% 22% 25% 8% 1	760
	21st to end	Night	25% 16% 27% 25% 6% 1	760
December	1st to 10th	Night	29% 17% 28% 20% 7%	760
	11th to 20th	Night	28% 16% 26% 27% 4	760
	21st to end	Night	25% 18% 28% 28% 2	836

Page generated at 2024-03-31 06:08:55. Version ac07

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